Aerosols

Aerosols are collections of tiny particles of solid and/or liquid suspended in a gas. The size of these particles can range from about 0.001 to about 100 microns. While a number of naturally occurring aerosols exist, the most familiar form of an aerosol is the pressurized spray can. Aerosols are produced by a number of natural processes and are now manufactured in large quantities for a variety of commercial uses. They are also involved in a number of environmental problems, including air pollution and destruction of ozone in the atmosphere.

Aerosols are commonly classified into various subgroups based on the nature and size of the particles of which they are composed and, to some extent, the manner in which the aerosol is formed. Although relatively strict scientific definitions are available for each subgroup, these distinctions may become blurred in actual practical applications. The most important of these subgroups are the following:

Fumes

Fumes consist of solid particles ranging in size from 0.001 to 1 micron. Some typical fumes are those produced by the dispersion of carbon black, rosin, petroleum solids, and tobacco solids in air. Probably the most familiar form of a fume is smoke. Smoke is formed from the incomplete combustion of fuels such as coal, oil, or natural gas. Its particles are smaller than 10 microns in size.

Dusts

Dusts also contain solid particles suspended in a gas, usually air, but the particles are larger in size than those in a fume. They range from about 1 to 100 microns (and even larger) in size. Dust is formed by the release of materials such as soil and sand, fertilizers, coal dust, cement dust, pollen, and fly ash into the atmosphere. Because of their larger particle size, dusts tend to be more unstable and settle out more rapidly than is the case with fumes, which do not settle out at all.

Mists

Mists are dispersions in a gas of liquid particles less than about 10 microns in size. The most common type of mist is that formed by tiny water droplets suspended in the air, as on a cool summer morning. If the concentration of liquid particles becomes high enough to affect visibility, it is then called a fog. A particular form of fog that has become significant in the last half century is smog. Smog forms when natural moisture in the air interacts with human-produced components, such as smoke and other combustion products, to form chemically active materials.

Sprays

Sprays form when relatively large (10+ microns) droplets of a liquid are suspended in a gas. Sprays can be formed naturally, as along an ocean beach, but are also produced as the result of some human invention such as aerosol can dispensers of paints, deodorants, and other household products.

Combustion aerosols

Aerosol technology has made possible vastly improved combustion systems, such as those used in fossil-fueled power generator plants and in rocket engines. The fundamental principle involved is that any solid or liquid fuel burns only at its surface. The combustion of a lump of coal proceeds relatively slowly because inner parts of the coal can not begin to burn until the outer layers are burned off first.

The rate of combustion can be increased by dividing a lump of coal or a barrel of fuel oil into very small particles, the smaller the better. Power-generating plants today often run on coal that has been pulverized to a dust, or oil that has been converted to a mist. The dust or mist is then thoroughly mixed with an oxidizing agent, such as air or pure oxygen, and fed into the combustion chamber. The rate of combustion of such aerosols is many times greater than would be the case for coal or oil in bulk.

Ozone depletion

A particularly serious environmental effect of aerosol technology has been damage to the Earth's ozone layer. This damage appears to be caused by a group of compounds known as chlorofluorocarbons (CFCs) which, for more than a half century, were by far the most popular of all propellants used in aerosol cans.

Scientists originally felt little concern about the use of CFCs in aerosol products because they are highly stable compounds at conditions encountered on the Earth's surface. They have since learned, however, that CFCs behave very differently when they diffuse into the upper atmosphere and are exposed to the intense solar radiation present there.

In those circumstances, CFCs decompose and release chlorine atoms that, in turn, react with ozone in the stratosphere. The result of this sequence of events is that the concentration of ozone in portions of the atmosphere has been decreasing over at least the past decade, and probably for much longer. This change is not a purely academic concern since Earth's ozone layer absorbs ultraviolet radiation from the Sun and protects animals on Earth's surface from the harmful effects of that radiation. For these reasons, CFCs have been banned from consumer product aerosols since the late 1970s. They are still employed for certain medical applications, but by and large they have been eliminated from aerosol use. The aerosol industry has replaced CFCs with other propellants such as hydrocarbon gases (e.g., butane and propane), compressed gases (e.g., nitrogen and carbon dioxide), and hydrochlorofluorocarbons (which are much less damaging to the ozone layer.)

Technological solutions

Methods for reducing the harmful environmental effects of aerosols such as those described above have received the serious attention of scientists for many years. As a result, a number of techniques have been invented for reducing the aerosol components of things like stack gases. One device, the electrostatic precipitator, is based on the principle that the particles of which an aerosol consists (such as unburned carbon in stack gases) carry small electrical charges. By lining a smokestack with charged metal grids, the charged aerosol particles can be attracted to the grids and precipitated out of the emitted smoke.

Another risk associated with commercial aerosols is their use as recreational drugs. Inhalation of some consumer aerosol preparations may produce a wide variety of effects, including euphoria, excitement, delusions, and hallucinations. Repeated sniffing of aerosols can result in addiction that can cause intoxication, damaged vision, slurred speech, and diminished mental capacity.